2003A12 Explain the mechanisms that maintain cerebral blood flow on moving
from a supine to a standing position.

 

List:

·        Intro: cerebral circulation and flow dynamics

·        Direct effect: cerebral and systemic

·        Compensation: cerebral and systemic

 

Intro:

Cerebral circulation

·   Normal CBF 750mL/min, 58mL/min/100g

·   Supply by internal carotid and vertebral arteries

·   Drainage by venous sinuses

·   Constant CBF vital; even brief interruption causes unconscious collapse
(risk of injury, vulnerability)

Flow dynamics

·   CBF = Cerebral perfusion pressure (CPP) / cerebral vascular resistance (CVR)

·   Starling resistor mechanism:

o In health: CPP = cerebral arterial – cerebral venous pressure

o If raised ICP: CPP = cerebral arterial – intracranial pressure

·   Normal values:

o Cerebral arterial = mAP when supine= 100mmHg at right atrial level

o Cerebral venous = CVP when supine = 2mmHg at right atrial level

o ICP = 5-10mmHg supine at external acoustic meatus

·   Resistance = (8 x length x viscosity) / π x radius4) (assuming laminar flow)

·   Hence vasodilation -> ↑flow

 

Direct effect: supine -> standing

Hydrostatic change

·   e.g. +30cmH2O = +22mmHg

CNS effect

·   10% ↓CPP, 10% ↓CBF (instantaneous)

o Cerebral arterial pressure: 100->78mmHg

o Cerebral venous pressure: 2->-20mmHg

o ICP: 5->-10 mmHg due to a) ↓venous>arterial volume b) ↓CSF volume

o CVR: unchanged

o CPP: (100 – 2) -> (78 - -10): 98 -> 88 (~10% drop)

CVS effect

·   ↓CPP, ↓and CBF (in seconds)

o mAP: venous pooling -> ↓preload -> ↓CO -> ↓mAP -> ↓CPP -> CBF

 

Compensation: supine -> standing

Aim

·   Attenuate the drop in CPP

·   Note: all responses impaired under GA

CNS effect

·   Myogenic autoregulation (instantaneous)

o ↓Stretch -> ↓reflex vasoconstriction -> ↓CVR -> ↑CBF

·   Metabolic autoregulation (in seconds)

o ↑H+, K+, lactate -> vasodilatation -> ↓CVR -> ↑CBF

·   Central ischaemic response (in seconds)

o ↑SNS -> ↑cardiac output, vasoconstriction -> ↑mAP -> ↑CBF

·   Fainting (in seconds)

o Loss of uphill hydrostatic gradient -> ↑CBF

CVS effect

·   Skeletal muscle pump and valves (in seconds)

o Activity -> ↑venous return, preload -> ↑cardiac output -> ↑mAP -> ↑CBF

·   Baroreceptor response (in 10-30 seconds)

o ↓Stretch of carotid sinus and aortic arch baroreceptors -> ↑SNS, ↓PSNS output -> ↑cardiac output, vasoconstriction -> ↑mAP -> ↑CBF

o Note delayed if elderly, autonomic neuropathy

 

(*Note: the “cerebral arterial pressure” and “cerebral venous pressure” terminology is my own)

 

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